linux-kernel - Re: + fs-epoll-add-procfs-fdinfo-helper.patch added to -mm tree

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Message-ID: <20121113170159.GW6511@moon>
Date:	Tue, 13 Nov 2012 21:01:59 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	James Bottomley <jbottomley@...allels.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + fs-epoll-add-procfs-fdinfo-helper.patch added to -mm tree

On Tue, Nov 13, 2012 at 05:49:51PM +0100, Oleg Nesterov wrote:
> On 11/13, Cyrill Gorcunov wrote:
> >
> >  struct signalfd_ctx {
> > +	rwlock_t lock;
> >  	sigset_t sigmask;
> 
> Oh, I don't think.
> 
> rwlock_t is horrible in general, and what it can buy for signalfd?
> A plain spinlock would be better. Or seqlock_t.
> 
> Whatever you do, you are trying to introduce the lock which should
> serialize the access to ->sigmask correctly. In this case I think
> you should split this change into 2 patches. The first one should
> fix the locking, imo. sys_signalfd4() should not use ->siglock at
> all, and the users which take ->siglock to read ->sigmask should be
> updated.

I see

> 
> Or,
> 
> > +#ifdef CONFIG_PROC_FS
> > +static int signalfd_show_fdinfo(struct seq_file *m, struct file *f)
> > +{
> > +	struct signalfd_ctx *ctx = f->private_data;
> > +	sigset_t sigmask;
> > +
> > +	read_lock(&ctx->lock);
> > +	sigmask = ctx->sigmask;
> > +	read_unlock(&ctx->lock);
> 
> Just read ctx->sigmask lockless. Do we really care if show_fdinfo()
> reads the value "in between" ?

As from c/r patch I think we can read it lockless (since we do stop
tasks anyway before doing checkpoint). So I would prefer to provide
it without locks at all.
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